In order to reduce the weight of vehicles (such as cars and the like), it has been proposed that hollow coil springs be used as spring parts (for example, Patent Document 1). The hollow coil spring of Patent Document 1 is produced by performing a hot hydrostatic extrusion step, a thickness reduction step, a rolling step, a coil forming step, a heating step, an end surface polishing step, a surface processing step, a setting step, and a coating step in turn. In the hot hydrostatic extrusion step, a seamless spring steel pipe is formed. In the hollow coil spring, a spring member (a seamless spring steel pipe or the like) is used, so that fatigue strength to torsional stress and bending stress can be improved.
Patent Document 1 is Japanese Unexamined Patent Application Publication No. 2007-127227.